请帮我把下面内嵌式html代码改成外部div+css形式

body>div { width: 600px; height: 400px; background-color: blue; } body>div:first-child { width: 450px; height: 400px; background-color: blue; float: left; } body>div:first-child >div { float: left; width: 200px;